I've had some bad experiences with pedals. The very first bike my parents bought for me was a red Schwinn with 20" wheels. In the first month or two I had 3 pedals fall apart and my father got them replaced under warranty. Finally the Schwinn dealer gave him a higher quality pair that lasted until I outgrew the bike. Then there was my Schwinn Varsity in college. Riding along about a month after I bought it the spindle of the left pedal snapped right in the middle. Fortunately I was able to keep control of the bike and only swerved a bit. A car behind me ran over the pedal so when I took it back to the LBS they at first refused to warranty it. I asked what they expected me to do - dive out in front of traffic to retrieve it? Finally got a warranty replacement. Didn't have any further pedal issues until clipless ones came along. I've had three SPD pedals fail as a result of problems in the click engagement mechanism and one had a sealed bearing disintegrate.
Can't answer the original question since some of my pedals have failed in the first week while others have lasted decades.